This invention relates to a processing system for microwave signals residing within a frequency band of interest wherein a microwave integrated circuit is mounted in a conductive housing and the housing is an integral part of the system. The conductive housing is provided with means to couple the input and output regions of the integrated circuit to the housing for microwave signals within the frequency band while providing dc isolation therebetween for the operating voltages of the active elements of the integrated circuit.
The increasing consumer acceptance and market for systems which receive microwave signals carrying television information from satellites in geosynchronous orbits can be noted by observing the ever increasing number of antennas throughout the country. A typical private receiving station includes a parabolic reflecting antenna which concentrates the received signal to a prime focus waveguide feed which serves as the input to the subsequent microwave processing systems.
The antenna concentrates the gathered signals within the 3.7 to 4.2 GHz band of interest with the centrally mounted waveguide input section assisting in this process. A conductive housing is located behind the waveguide input section and contains the processing systems for the received microwave signals. The first system is typically a low noise amplifier mounted in a conductive housing and contains a waveguide probe at its front end to couple the received signals into the processing systems. This initial amplifier system is normally provided with a coaxial output connection to subsequent processing systems which include the converter for lowering the frequency range to 70 MHz and the receiver for further signal processing prior to being fed to the home display equipment.
The first processing system, i.e., the low noise amplifier system, normally includes a conductive housing with a microwave integrated circuit fastened to a large area surface of an enclosed chamber. A number of different systems are presently in use, many containing an isolator or circulator interposed between the waveguide coupling probe and the integrated circuit for reducing the noise performance of the system due to reflections and standing-wave interactions. The output signal from the isolating section is supplied to the integrated amplifier circuit. The amplifier signal is then fed via coaxial connector to subsequent processing systems.
The operating voltage for the active elements of the microwave circuit is supplied through the coaxial cable which establishes a dc voltage difference between the conductive housing and the strip conductor network located on the top surface of the integrated circuit. In practice, the integrated circuit is attached to the chamber of the housing by threaded fasteners which ensure good electrical contact between the housing and the ground plane of the integrated circuit. The magnitude of the dc voltage supplied is determined by the voltage drops and line losses encountered before reaching the processing system. These voltage drops vary so that regulation is provided on the integrated circuit to insure a constant voltage supply. The regulated output voltage, for example +5 volts dc with respect to the ground plane and conductive housing, is then utilized by an integrated circuit mounted within the housing. A separate integrated circuit, e.g., an inverter, generates the negative bias supply voltages for the active devices in the processing system.
The microwave integrated circuit, as the term is used, is a hybrid form of integrated circuit with individual field effect transistors mounted on a dielectric substrate having a strip conductor network on one surface thereof and a ground plane on the opposing surface. The regulator and inverter circuits along with discrete passive elements used therewith are mounted either on the segment of microstrip transmission line as individual components or on a separate board mounted within a separate cavity remote from the microstrip segment. The additional assembly steps required for the mounting of the inverter integrated circuit and the cost of these additional components add to system costs and complexity but have heretofore been needed to establish negative voltage levels for the type of microwave transistors favored for use in these systems.
The field effect transistor (FET) presently utilized for microwave signals is an N-channel device operated in the depletion mode. This type of operation requires a negative gate-to-source bias. The favored material for microwave FETs is gallium-arsenide and due to the characteristics of the material and the devices, self-bias techniques employing resistors and capacitances in the input signal path which are useful at lower frequencies and stronger signals cannot be employed without increasing the system noise figure to unacceptable levels. Consequently, the additional components have heretofore been required for the microwave integrated circuit in order to establish negative bias voltages.
Accordingly, the present invention is directed to the provision of a system for processing microwave signals wherein the ground plane of the microwave integrated circuit is electrically coupled to the conductive a housing for signals within the bands of interest but isolated therefrom for bias voltage purposes. Consequently, the ground plane voltage of the integrated circuit can be determined independently of the housing voltage, and a negative gate-source bias voltage established without requiring the addition of an inverter circuit to the system.
The elimination of the integrated circuit inverter and the supporting strip conductor network reduces the complexity and cost of the microwave integrated circuit thereby providing a simplified, less expensive system for processing microwave signals.